Master Python Debugging: A Hands‑On Guide to Using Pdb Effectively

Introduction to Pdb

Python includes a built‑in debugger, Pdb, which allows setting breakpoints, stepping through code, and inspecting variables.

How to launch Pdb

Insert a breakpoint in the code

Place import pdb; pdb.set_trace() at the desired line.

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n-1)

import pdb; pdb.set_trace()
print(factorial(5))

Start from the command line

Run a script with the -m pdb option:

python -m pdb myscript.py

Basic Pdb commands

b

( break): set a breakpoint c ( continue): continue execution until the next breakpoint s ( step): step into functions line by line n ( next): execute the next line without stepping into functions p ( print): evaluate and display an expression q ( quit): exit the debugger

Practical example

Script that computes the average of a list:

def average(numbers):
    total = sum(numbers)
    count = len(numbers)
    return total / count

numbers = [1, 2, 3, 4, 5]
print(average(numbers))

Set a breakpoint and start debugging

Insert import pdb; pdb.set_trace() before the function call:

import pdb; pdb.set_trace()

def average(numbers):
    total = sum(numbers)
    count = len(numbers)
    return total / count

numbers = [1, 2, 3, 4, 5]
print(average(numbers))

Running python -m pdb myscript.py pauses at the breakpoint, allowing step‑by‑step execution.

Inspect variable values

Use the p command to display a variable:

(Pdb) p numbers
[1, 2, 3, 4, 5]
(Pdb)

Advanced features

Pdb also supports conditional breakpoints and stack inspection.

View the call stack

Use the where command. Example output:

(Pdb) where
  <frozen runpy>(198)_run_module_as_main()
  <frozen runpy>(88)_run_code()
  c:\src\uml\2024\07\myscript.py(9)<module>()
  -> print(average(numbers))

Conclusion

Mastering Pdb’s commands and advanced features can significantly speed up debugging of Python programs.